Let’s say you knew it was going to rain and you even saw the storm clouds on the horizon. But you decide you are going to wash your car while it is raining. Something like that would seem like a waste of time and labor. Like having an exercise class where you try to touch the ceiling during jumping jacks: https://www.cartoonstock.com/cartoonview.asp?catref=smb080129. Paul strives to emphasize the futility and meaninglessness of the Christian life (and life in general) if there is no resurrection. The honest truth is that if Jesus hadn’t been raised from the dead, then Christianity is pointless and is a waste of time (1 Corinthians 15:13-15).